The present invention relates to a molding system for producing articles, such as preforms and tubular parts, and to an innovative rotary cooling station employed in said system. The present invention further relates to a method for producing molded articles.
Reduction of the injection molding cycle time is a major task when forming articles in a huge volume. This is for example the case of PET preforms that are formed using high cavitation molds, such as for example the 72 or 96 cavity molds made by Husky Injection Molding Systems, the assignee of the instant application.
One option to reduce the molding cycle time is to limit the residence time of the preforms in the mold closed position by shortening the cooling step by a few seconds and thus ejecting the preforms from the mold sooner. One immediate benefit of a shorter residence time is that the content of acetylaldehyde (AA) in the final preform will drop by at least 5%. Because the preforms are still warm and fragile when they are transferred out of the mold to a robot having a take-out plate, they are vulnerable to deformation and surface damage. The internal heat retained by the preforms generates unacceptable crystallinity zones, especially at the sprue gate and neck finish portion of the preforms, if sufficient cooling is not provided immediately after opening the mold. Accordingly, optimizing and fine tuning the post mold cooling process and developing the necessary hardware is difficult when molding time is pushed beyond the normal cycle times.
Numerous attempts have been made in the past to improve the post-mold cooling process when forming PET preforms. These methods and equipment are not particularly applicable to a very fast molding cycle and do not properly cool the preforms ejected from the mold. Because early ejected preforms have warmer walls and retain a warmer air pocket inside the preform's inner cavity than the preforms made under normal conditions, they have to be cooled as soon and as fast as possible to prevent formation of crystallinity spots, or surface damage and deformation. Reference is made in this regard to U.S. Pat. No. 4,449,913 which shows an injection molding machine with a four face rotary mold plate, each face carrying a mold core plate. After molding, the preforms are removed from the cavities and retained on the injection cores for further internal cooling and for external cooling done by air blowers. The post-molding cooling step is performed while a new batch of preforms are formed on the same machine. This cooling approach is quite effective but the equipment is very expensive and complicated because three additional mold plates and three additional sets of injection cores are needed. This increases by three times the weight of the rotary mold plate block relative to a single face mold. In addition, these four mold plates and injection cores have to be manufactured with higher tolerances than those needed to make a single mold plate. This is due to the alignment requirements of the four sets of rotary cores with respect to a single stationary set of mold cavities. Also the relatively low speed of rotating, aligning and translating the heavy rotary mold core plate is a factor that significantly increases the molding cycle.
U.S. Pat. No. 4,836,767 to Schad et al. relates to an injection molding machine with a rotary mold core plate having two sets of mold cores. This is a dedicated nonstandard molding machine that has only three tiebars, one being used as a rotation axis for the mold core plate. After molding one batch of articles, such as PET preforms, they are retained on the mold cores and the mold core plate is rotated by one tiebar. In this way, the preforms are removed from the molding area and then are ejected into a rotary cooling station. This four face cooling station includes tubes that retain and cool the preforms from the outside. After being externally cooled, the preforms are ejected and dropped with the neck finish downward on a conveyor. If the preforms are not sufficiently cooled the neck finish can be damaged. This combination of a rotary mold machine and a rotary cooling station is much slower than having a dedicated robot to remove the preforms from the mold. This patent does not teach or anticipate internal cooling by the rotary cooling station or simultaneous external and internal cooling. Internal cooling in addition to the external cooling they teach would require a specialized equipment and highly accurate alignment.
U.S. Pat. No. 5,569,476 to Manen and Albers relates to a standard injection molding and a four face rotary cooling station. Each face of this rotary cooling station is movable and thus functions as a take-out robot because it can be moved laterally between the mold plates to remove the preforms and bring them to the cooling station. This design has several drawbacks: (a) the preforms are cooled solely externally on the retaining tubes belonging to the rotary cooling station; (b) the system is expensive and complicated as it must use four rather than just one robot arm that travel for a relatively long distance. This patent does not teach or suggest internal cooling, does not teach or suggest simultaneous internal and external cooling, and does not teach or suggest a single robot that feeds a rotary cooling station.